Arrangement in safety switches for microwave stoves



March 31, 1970' B. A.HORNER ,50

' ARRANGEMENT IN SAFETY'SWITCHES FOR MicRowAvE s'rovms Fild-Aug 28, 1968 '2 Sheets-Sheet l f a k \X \N INVENTOR. BROR A. HORNER BY iaw l9.

March 31', 1970 v 'Q BLALHORNE R 3,504,1

I ARRANGEMENT IN SAFETY SWITCHES FOR MICROWAVE STOVES Fild Aug. 28. 1968- 2 Sheets-$heet 2 &\\\\

INVENTOR. BROR A. HORNER BY M P AGENT United States Patent 3,504,144 ARRANGEMENT IN SAFETY SWITCHES FOR MICROWAVE STOVES Bror Alex Homer, Vallingby, Sweden, assignor, by mesne assignments, to US. Philips Corporation, New York,

N.Y., a corporation of Delaware Filed Aug. 28, 1968, Ser. No. 755,992 Int. Cl. H01h 3/16 US. Cl. 20061.62 4 Claims ABSTRACT OF THE DISCLOSURE A door operated safety switch for a microwave oven comprising a first arm rotatable with the door and having a first camfollower contacting a concave cam guidance surface on a second pivotally mounted arm that actuates the safety switch. The cam guidance surface is shaped so that, with the door closed, the second arm does not actuate the switch, but when the door is initially opened, the cam follower contacts a projecting portion of the guidance surface so that the second arm actuates the safety switch. The cam surface is further shaped so that as the door is opened further, the second arm holds the safety switch in the actuated position. The lever ratio of the second arm is chosen to have a value appreciably greater than one. A second cam guidance system can be interposed between the free end of the second arm and the safety switch.

This invention relates to an arrangement for actuation of a safety switch in microwave stoves in dependence upon the opening movement of the door of the stove. Such a safety switch is usually used in combination with a micro-switch which is also actuated by the door. These two switches will together ensure that high frequency energy will not be applied to the cavity of the stove when the door is wholly or partly open. The micro-switch is included in the control circuits of the magnetron and operates at low power levels. If the door is opened by mistake while the magnetron is still in operative condition, it is the micro-switch which is initially effective to interrupt the high frequency supply. The switching circuits controlled by the micro-switch may, however, be ineffective for example, by welding of the contact means due to repeated switching. The said safety switch is then arranged as a further safety measure for preventing the supply of energy to the cavity when the door is opened. On this safety switch there is laid the requirement that it shall be compulsorily controlled by the door so that the door cannot be opened without actuating the safety switch for interrupting the energy supply to the cavity. The safety switch can, for example, be connected in the electric supply circuits of the magnetron whereby it will operate at high power levels. Even if it is normally not adapted to interrupt the energy supply to the cavity it may, however, under unfavourable circumstances get such an interrupting function and it must therefore be controlled such that it will interrupt the current before appreciable energy quantities have leaked out through the gap between the door and the edges round the cavity at the opening of the door. With the great energy quantities appearing in the cavity this means that the safety switch must function already at very small opening angles of the door.

It has previously been proposed to achieve the compulsory coupling between the door and the safety switch by a link which is pivotally connected to the door at one edge of the same and which link under the first portion of the opening movement of the door is directly coupled with the actuation means of the safety switch. During the continued opening movement, the link will be decoupled from the safety switch. This results in a ICC bulky and complicated device which furthermore has the drawback that the said link will limit the free space for handling the food. By the present invention, an actuation and coupling device for the safety switch is achieved which is compact and which can be made quite invisible from outside and which in spite of this can bring the safety switch to function for extremely small movements of the door. The characteristic of the invention consists in that the safety switch is coupled with the door through a cam guidance device consisting of a projecting arm arranged at the pivot axis of the door and rotatable with the door. This arm is provided with follower means cooperating with a concave cam guidance surface arranged on a pivotally mounted arm which is coupled with the activating means of the safety switch, the said concave cam guidance surface being shaped such that with the door closed, the pivotal arm will assume a position in which the safety switch is unactuated. When the door is initially opened, the said follower means will run against a projecting portion of the cam guidance surface so that the pivotally mounted arm will be rotated and cause actuation of the safety switch. During the continued opening movement, the pivotal arm is maintained in the switch actuating position. The lever ratio of the pivotally mounted arm, i.e. the ratio between the distance from the rotation axis to the point of actuation of the safety switch and the distance from the same axis to the said projecting portion, is appreciably larger than 1 for achieving a rapid actuation of the safety switch.

The main advantage of this inverse cam guidance is that the cam guidance surface, which must have an angular extension corresponding to the whole opening angle of the door, or approximately will be situated at'a distance from the pivot axis of the door and can be placed within the stove so that it is quite invisible from outside. The device can furthermore easily be shaped such that even a small force exceeding the normal force required for actuation of the safety switch, for example, caused by welding of the contacts, will automatically cause effective locking of the door so that it cannot be opened.

The invention is illustrated in the accompanying drawings, which show a device for actuation of a safety switch in microwave stoves in which:

FIG. 1 is a fragmentary plan view of an oven with the top surface removed and the oven side walls and the oven door shown in cross-section, and

FIG. 2 is a fragmentary front elevation with the top and one side wall shown in section and a portion of the door shown pivotable about its support shaft.

- In the drawings reference numeral 1 designates the door of a microwave stove, the cavity of which is indicated by reference numeral 2 and the outer envelope by reference numeral 3. The door is pivotally mounted at its lower edge on a supporting shaft 4 secured in the stove. In the space between the outer envelope 3 and the limit wall of the cavity there is a safety switch 5 mounted on a plate 6. The plate 6 is in turn fixed to a base plate 9 by means of screws 7, 8. The base plate 9 may in turn be fixed to a wall of the oven by means of a-bracket, etc., not shown. The switch 5 is assumed to be connected in the supply circuits of the magnetron such that the energy supply to the cavity is interrupted when arm 10, at its free end'has a' rotatably mounted roller-12 co-operating with a concave cam guidance surface 13 on an arm 14. The arm 14 is pivotally mounted on the plate 6 so that it is rotatable about a shaft 15. At its free end the arm 14 is shaped so as to form a second cam guidance surface 16 which, in co-operation with a roller 17 mounted on a swingable arm 18, formsthe second cam guidance device. The arm 18 is pivotally mounted on the plate '6 by means of a shaft 19. The actuation means 20 of the safety switch bears against the arm'18 so that the switch will function upon a certain deflection of the said arm. A spring 21-is connected to a perpendicularextension of the arm 14 so as to hold the cam guidance surface 13 pressed against the roller 12, while the roller 17 is pressed against the cam guidance surface 16by the spring of the actuation means 20. The free end of spring 21 may be fixedin position by connecting same to a bracket or other suitable fixture, not shown.

The door is shown in the closed position, the two cam guidance devices being in their initial position and the switch being unactuated. When the door is opened the arm will be swung in a counterclockwise direction, whereby the roller 12, after initially lying in arecess 13a, runs against a projecting portion or a cam 13b of the cam surface. Thereby the arm 14 will be influenced such that it is swung in a clockwise direction. During any further opening motion of the door 1, the roller will run along a portion 130 which is situated substantially at the same radius as the cam 13b, wherebythe arm 14 is held in the switch actuating position. At the upper carn guidance de vice the cam function is produced in that the roller 17 is initially only partly rolled onto the surface 16. As soon as the arm 14 is swung in the clockwise direction, the arm 18 will be pressed upwardly by cooperation between the roller 17 and the surface 16, the switch 5 being actuated at a certain deflection of the arm 18. The output/ input motion ratio, i.e. the ratio between the linear output movement of the actuator means and the input movement or the opening angle of the door, is determined on the one hand by the shape of the cams and on the other hand by the lever ratio of arm 14. In order to obtain a high motion ratio, the cams are shaped such that they have a high slope at the start of the motion and the said lever ratio is made several times larger than 1, in the example shown approximately 5. The cams are furthermore adjusted such that both arms 14 and 18 are coupled to the door without play so that they start to move immediately at the opening of the door. In spite of a compact construction and small dimensions of the device, it can hereby be ensured that the linear motion of the means 20 will be larger than the motion of the door at the opening edge of the same.

The device is also irreversible in that the door is locked effectively if the contacts in the switch 5 for some reason should be welded together.

What is claimed is:

1. A structural combination for actuating a safety switch having actuation means in amicrowave stove with the opening movement of ;the stove door. comprising, cam guidance means for coupling the switch to the door and including a projection arm arranged at the supporting shaft of the door and coupled to the door so as to be rotatable therewith, a first arm pivotally mounted for rotation about a given axis and coupled to the actuation means of the safety switch, said pivotally mounted arm having a concave cam guidance surface with a projecting portion, said projecting arm having a follower means cooperating with said concave cam guidance surface, the concave cam guidance surface being shaped such that, with the door closed, the pivotally mounted arm assumes a position in which the safety switch is unactuated, while at the initial opening of the door said follower means runs against the projecting portion of the cam guidance surface so that the pivotally mounted arm is rotated to actuate the safety switch and is thereafter maintained in the switch actuating position during any further opening motion of the door, the lever ratio of the pivotally mounted arm being appreciably greater than one to produce rapid actuation of the safety switch, said lever ratio being defined as the ratio between the distance from the rotation axis of the pivotally mounted arm to the point of actuation of the safety switch and the distance from the same axis to the said projecting portion.

2. A combination as claimed in claim 1 wherein said pivotally mounted arm has a free end, said guidance means further comprises a cam guidance device situated at the free end of said pivotally mounted arm for coupling said arm to the actuation means of said safety switch.

3. A combination claimed in claim 2 wherein said cam guidance device comprises, a pivotally mounted arm having a cam follower mounted thereon and in contact with a second cam guidance surface on said first pivotally mounted arm.

4. A combination as claimed in claim 1 wherein said cam guidance surface includes an arcuate portion having a radius equal to the distance between the door shaft and the cam follower means of said projecting arm.

References Cited UNITED STATES PATENTS 868,348 10/1907 Kramer 200-61.62 2,882,360 4/1959 'Sisson 200-61.62 XR FOREIGN PATENTS 710,922 6/1965 Canada.

HERMAN O. JONES, Primary Examiner M. GINSBURG, Assistant Examiner U.S. Cl. X.R. 219l0.55 

